1. Field of the Invention
The present invention relates to a method of marking a metal device with an identification symbol or a serial number that is not detectable by a visual or microscopic examination and more particularly to a method of marking a metal device which changes the surface microstructure in the shape of the identification symbol or serial number.
2. Description of the Prior Art
In the past various methods have been used to mark metal devices for the purposes of services and maintenance, trademark usage, theft prevention and quality control with a stamped or engraved identification symbol or serial number. Some of these methods of marking include engraving, stamping, etching and casting. The purposes of the identification symbol or serial number are generally achieved by these methods of marking. However, in the case of theft of the metal device where a thief may obliterate the identification symbol or serial number, the purpose of marking is not achieved.
There are many techniques which have been used to place inconspicuous identification symbols and serial numbers on metal devices. These techniques include high resolution laser engraving and photo-etching. Each of these techniques leaves a mark which a thief of the metal device can find by a visual or microscopic examination. If the thief finds the mark, he will obliterate it. Furthermore, wear will also obliterate these identification symbols and serial numbers.
U.S. Pat. No. 3,588,439, entitled High Resolution Laser Engraving Apparatus, issued to Mary E. Heller and Hendrik J. Gerritsen on June 28, 1971, teaches a pulsed laser which illuminates an entire given area of the surface of a member to be engraved with coherent light which varies in relative intensity from point to point over its cross section in accordance with a predetermined pattern. It has been found that if the absolute intensity and time of duration of the pulse of coherent light are proper, it is possible to obtain engravings in which details of the order of 2 microns in the engraved pattern can be resolved. This makes it possible to engrave not only pictorial patterns, but also hologram patterns.
U.S. Pat. No. 4,128,752, entitled Laser Micromachining Apparatus, issued to Joseph A. M. Gravel on Dec. 5, 1978, teaches a laser micromachining apparatus which is capable of producing a particular machining pattern on a workpiece. The laser micromachining apparatus includes a laser device for producing a laser beam, a beam expander unit positioned to receive the laser beam and transforms it into a beam of larger and substantially uniform cross-sectional area and a mask device in the path of the laser beam after the beam expander unit. The mask device includes a diffraction pattern which corresponds to the particular machining pattern and which includes a plurality of different individual perforations. The apparatus also includes a converging lens system which focuses the laser beam and which is positioned adjacent to the mask device in the path of the beam. The laser micromachining apparatus also includes image optics in the path of the laser beam after the converging lens and system and the mask device for imaging the whole of the diffraction pattern of the mask onto the workpiece. The converging lens is a spherical lens superposing the diffraction pattern of the different individual perforations of the mask at the entrance pupil of the image optics to provide a minimum size imaging unit for reproducing the diffraction pattern on the workpiece.
U.S. Pat. Nos. 4,128,753 and 3,617,702 are other patents which teach laser apparatus for use with metal devices.
The technique of photo-etching includes the steps of coating the surface of the metal device with a photo-resistance material, covering the surface with a mask which has the identification symbol or serial number exposed, shining a light onto the mask thereby exposing the unmasked surface area, and using a chemical etch which marks the surface of the metal device.